I live near Boston. To clarify, delivery drivers do get paid but not much. It’s a job like hairdresser, waitron, or newspaper delivery: tips are a significant part of their income.
I imagine that some portion of a restaurant delivery charge goes toward what it pays its drivers, but certainly the percentage will vary according to the owners and whether it’s an independent place or a member of a chain. I tip 15-20% of the cost of the order, more if the weather is bad. For grocery delivery, $5-10, depending not on the total cost but on the number and weight of the bags, and the weather.